The production of high-purity dimethyl ether (DME) is a commercially-important process. Typically, DME is formed by dehydration of methanol (MeOH) in a DME reactor system.
The feed to a DME reactor system is methanol, which usually contains a certain amount of water and other impurities (e.g. other organic alcohols, ketones, etc.). Water is an undesirable inhibitor of the dehydration to DME.
The DME obtained from a DME reactor system is typically in vapour form, and also contains unreacted methanol, water, and impurities (both those which were present in the methanol feed and those generated by the DME reactor system itself).
The vaporous DME is cooled and then subjected to column distillation, which provides an offgas stream of light fractions (e.g. CH4, CO2, N2), DME product and a stream containing unreacted MeOH and water. Unreacted MeOH and water are recycled, the water is separated, and the methanol re-enters the system after purification.
Through such methods, a purity of over 99% DME can be obtained. However, for certain purposes (e.g. medicinal uses or personal care), extremely high purity (>99%) is required. The higher the initial purity of the DME, the more difficult it proves to obtain even a small increase in purity. This difficulty is further compounded by the presence of the impurities—especially the light fractions—and the liquid-vapour equilibrium constant of DME.
JP 2005-298457 discloses a method for producing DME in which methanol is obtained from synthesis gas.
U.S. Pat. No. 6,548,856 discloses a separation process for one-step production of DME from synthesis gas.
U.S. Pat. No. 5,750,799 discloses a process for the production of DME and recovery from methanol.
Other patent documents in the field of DME production and purification include U.S. Pat. No. 4,560,807, U.S. Pat. No. 5,684,213, DE 42 22 655, JP 2004-091327 and CN 1377871.
US 2011/0065963 discloses one process for providing high purity DME. The process involves a gas/liquid separator, in combination with an absorption column, receiving absorption liquid comprising water and methanol from a DME rectifier.
There remains a need for a process and apparatus which can provide DME with improved purity and efficiency over the processes of the prior art.